The present invention relates to an improved input circuit for a digital phase locked circuit, and more particularly relates to an improvement in the function of an input circuit for a digital phase locked loop used for detection of displacement of a mobile object via A/D (analog-digital) conversion.
A wide variety of detection circuits are currently proposed to detect the displacement of a mobile object through modulation of output signals from a detection encoder.
One example is proposed in Japanese patent application Sho. 61-54288 in which the detection circuit includes a digital phase locked loop. More specifically, an encoder issues analog signals sin.theta. and cos.theta. corresponding to the angular displacement .theta. of a mobile object. These analog signals sin.theta. and cos.theta. are then properly digitalized. A function generation ROM is separately provided to issue signals sin.phi. and cos.phi. on the basis of its address data. These signals sin.phi. and cos.phi. are multiplied with the digitalized signals sin.theta. and cos.theta., respectively. The products of multiplication are then compared with each other for calculation of a value sin(.theta.-.phi.). When the value sin(.theta.-.phi.) is positive, a signal U/D at "0" level is issued. Whereas a signal U/D at "1" level is issued when the value sin(.theta.-.phi.) is negative. The signal U/D is then passed to a counter which also receives a series of prescribed clock pulses. Upon receipt of the signal U/D at "0" level, the counter operates in an up-count mode. In contrast, the counter operates in a down-count mode on receipt of the signal U/D at "1" level. Output signals from the counter are properly latched in order to remove fluctuations in output.
In the case of such a detection circuit, the digital phase locked loop operates to ensure that the value sin(.theta.-.phi.) is equal to 0, i.e. .theta. is equal to .phi.. In other words, the output signal from the counter corresponds to the angular displacement .theta. of the mobile object.
As is well known, the analog signals sin.theta. and cos.theta. occasionally include noise generated by the presence of disturbances and such noise cannot be removed during A/D conversion. As a consequence, the digitalized signals sin.theta. and cos.theta. are poor in stability due to the inclusion of such noise. In particular, even when the mobile object moves slowly the stability is degraded under the influence of high range noise due to the relatively wide frequency bands of converters used for the A/D conversion.